1. Field of the Invention
This invention relates to the compression of video signals.
2. Description of the Prior Art
Compression of video signals on an intra-image basis (for example, compression on an intra-field or intra-frame basis) makes use of the redundancy present in pictures or images represented by the signals to reduce the amount of information needed to represent the pictures or images. The compression can be used to reduce bandwidth, in the case of transmission of a video signal, or to reduce storage capacity, in the case of storage of a video signal.
Intra-image compression can, as is known, be effected in the time domain by the use of differential pulse code modulation, in which a predictor is used to predict the values of samples representing pixels based on previous pixel values. Since the image pixels are highly correlated, the prediction is accurate and results in a small and uncorrelated error (that is, a difference between the predicted and actual values). The error samples are encoded and, since they can be encoded using fewer bits than the samples representing the original pixels, compression can be achieved.
FIG. 1 of the accompanying drawings shows a known apparatus or system for effecting intra-image compression of a video signal in the two-dimensional spatial frequency domain. A video signal, which is in digital form and comprises successive multi-bit (for example 8-bit) samples or words each representing a respective pixel of an scanned image or picture, is applied via an input 10 to a decorrelator 12. A decorrelated version of the video signal is outputted by the decorrelator 12 to a quantizer 14 and then to an entropy encoder 16, which together compress the decorrelated version of the video signal outputted by the decorrelator 12 to produce a compressed signal of an output 18. The compressed signal can then be transmitted or stored. (Note that, although the decorrelator 12, quantizer 14 and entropy encoder 16 are shown for clarity as being separate items, they may in practice be embodied in an at least partially combined form.) After transmission or storage, the compressed signal can be restored substantially to its original form by expansion by way of entropy decoding, dequantizing and correlation operations which employ parameters converse to those used for decorrelation, quantization and entropy encoding, respectively, upon compression.
The operation of decorrelation performed in the decorrelator 12 relies upon the fact that neighboring pixels of an image are highly correlated, whereby processing an image (for example, a field or frame of a video signal) to form decorrelated signal portions representing different components of the image in the two-dimensional spatial frequency domain enables a reduction in the amount of information needed to represent the image. Specifically, the decorrelated signal portions represent different spatial frequency components of the image to which the human psychovisual system has respective different sensitivities. The different decorrelated signal portions are subjected to different degrees of quantization in the quantizer 14, the degree of quantization for each signal portion depending upon the sensitivity of the human psychovisual system to the information in that portion. That is, each of the decorrelated signals is quantized in accordance with its relative importance to the human psychovisual system. This selective quantization operation, which is a lossy operation in that it involves deliberate discarding of some frequency data considered to be redundant or of little importance to adequate perception of the image by the human psychovisual system, in itself enables some signal compression to be achieved. The quantizer 14 enables compression to be achieved in two ways: it reduces the number of levels to which the data inputted to it can be assigned, and it increases the probability of runs of zero value samples on the data it outputs. Note that, in video signal compression apparatus described in detail below, the ability to achieve signal compression provided by the operation of the quantizer 14 is not used to produce a bit (data) rate reduction in the quantizer itself. Instead, in that case, the ability to achieve signal compression provided by the operation of the quantizer is carrier into effect in the entropy encoder 16 in that the reduction in information content achieved in the quantizer 14 enables a consequential bit (data) rate reduction to be achieved in the entropy encoder.
Further (non-lossy) compression, and bit (data) rate reduction, is provided in the entropy encoder 16 in which, in known manner, using for example variable length coding, the data produced by the quantizer 14 is encoded in such a manner that more probable (more frequently occurring) items of data produce shorter output bit sequences than less probable (less frequently occurring) ones. In this regard, the decorrelation operation has the effect of changing the probability distribution of the occurrence of any particular signal level, which is substantially the same as between the different possible levels before decorrelation, into a form in which in which it is much more probable that certain levels will occur than others.
The compression/coding system or apparatus as shown in FIG. 1 can be embodied in a variety of ways, using different forms of decorrelation. An increasingly popular form of implementation makes use of so-called transform coding, and in particular the form of transform known as the discrete cosine transform (DCT). (The use of DCT for decorrelation is in fact prescribed in a version of the compression system of FIG. 1 described in a proposed standard prepared by JPEG (Joint Photographic Experts Group) and currently under review by the ISO (International Standards Organization).) According to the transform technique of decorrelation, the signal is subjected to a linear transform (decorrelation) operation prior to quantization and encoding. A disadvantage of the transform technique is that, although the whole image (for example, a whole field) should be transformed, this is impractical in view of the amount of data involved. The image (field) thus has to be divided into blocks (for example, of 8.times.8 samples representing respective pixels), each of which is transformed. That is, transform coding is complex and can be used on a block-by-block basis only.
A recently proposed approach to compression/coding in the frequency domain is that of sub-band coding. In this approach, the decorrelator 12 in the system of FIG. 1 would comprise a spatial (two-dimensional) sub-band filtering arrangement (described in fuller detail below) which divides the input video signal into a plurality of uncorrelated sub-bands each containing the spatial frequency content of the image in a respective one of a plurality of areas of a two-dimensional frequency plane of the image, the sub-bands then being selectively quantized by the quantizer 14 in accordance with their positions in the sensitivity spectrum of the human psychovisual system. That is, decorrelation is achieved in this case by putting the energy of the overall image into different sub-bands of the two-dimensional spatial frequency domain. Sub-band filtering is believed to provide better decorrelation than the transform approach. Also, unlike the transform technique, there is no restriction to operation on a block-by-block basis: the sub-band filtering can be applied directly to the video signal.
The proposed JPEG standard mentioned above requires that data decorrelated by means of a block-by-block DCT transformation operation be quantized and then entropy encoded. The data is required by the standard to be quantized and encoded in a particular sequence dictated by the order in which it is outputted by the decorrelator, which order is in turn dictated by the way in which the input digital video signal is divided into blocks for transformation. The sequence is such that each successive one of a sequence of (for example) 8.times.8 arrays (blocks) of data each resulting from transformation of an 8.times.8 block of data of the input signal is, after quantization, entropy encoded in such a manner that one of the 64 data in the array is entropy encoded in a manner different than the other 63 data in the array. This requires switching of the entropy encoding at the block frequency, that is once every 64 data (samples). Moreover, the sequence is such that the 64 data in each block (in turn) are quantized and encoded in a particular order specified in the standard. In the case of sub-band filtering, the data to be quantized and encoded is of a very different format than that obtained in the case of block-by block DCT transformation. On the face of it, the JPEG standard approach not only excludes the use of sub-band filtering, but, on the face of it, is wholly incompatible with the use of sub-band filtering, which does not require a block-by-block approach. This is unfortunate because, as mentioned above, sub-band filtering is believed superior to block-by block DCT transformation.